Electrolytic refining of copper results in the formation of an anode mud or slime which will range anywhere from about 0.5 to 3% or more of the original anode weight. The composition of the slime will vary dependent upon the components of the anodes which are not soluble in electrolyte. The slimes themselves are of fine particle size, about -200 mesh, and ordinarily grayish-black in color. Slimes usually contain varying quantities of copper, silver, gold, sulfur, selenium, tellurium, antimony, arsenic, nickel, iron, silica, bismuth, and the like. Ordinarily, the largest component of the slimes is copper, which ranges from about 15 to 30% by weight, or more, of the slime. In addition to the valuable copper present in the slimes, there is a significant amount of precious metals, such as silver and gold, as well as other metals which are of economic value, such as selenium and tellurium.
Because of the large amounts of copper in the slime as well as of the precious metals and other valuable metals, a variety of techniques have been utilized to recover such metals for the slime. None, however, have been commercially successful because of the large cost involved and the fact that it is difficult to recover the precious metals from the slime. One type of technique involves direct smelting of slimes. This is disadvantageous since excessive formation of matte and slag cause heavy recirculation of precious metals, especially of silver. Such techniques involve roasting to convert copper to copper oxide followed by leaching of the roasted slimes with sulfuric acid. The leached slimes are then melted down in a small reverberatory (dore) furnace in which the impurities are oxidized and to leave a dore bullion of the precious metals silver and gold. The dore is then parted to recover the fine gold, fine silver, and any other precious metal present in the original slime. This particular technique is, as noted, unsuitable because of the excessive formation of matte and slag.
A variety of other pyrometallurgical techniques combined with acid leaches and acid leach procedures have also been attempted, but they equally are not suitable mainly because it has not been possible to initially remove substantially all of the copper from the slime. It has been found that copper levels at or above about 5% by weight of the material to be treated greatly inhibit the known procedures for recovery of selenium and precious metals, particularly silver, from slime.
In addition, in leaching slimes with an acid, even strong sulfuric acid leach solutions (400 g/l), a leach time under agitation of up to 72 hours is required. Also, during such leach, it is necessary to periodically stop the agitation, heat, and aeration to permit the solids to settle and then to decant the clear solution. After decantation, additional leach solution (water and acid) is added and heat, air, and agitation started to continue the leach.
As a consequence, it has heretofore been costly in terms of material, labor, maintenance, and time to recover the copper, precious metals, and other metal values from copper anode slimes.